It seems like the verticalBoxSizer->Add( grid, 1, wxEXPAND ) overwrites the grid->SetMaxSize when resizing the window. The grid is stretched so that the white Space is still there.
Unfortunately, there is no "wxFIXED_MAXSIZE" that i could use instead of wxEXPAND. There only is a wxFIXED_MINSIZE, but that does not help me much.
My furthers attempts also did not show good result.
It seems to me like there is no possibility to do what i want without catching the resizing event and doing everything on my own.